I try to :
a- change the background color of the sequence flow => only the triangle's color is changed, the line part is transparent
b- change the border color of the sequence flow => the border of the triangle is displayed with the selected color and the line part the SequenceFlow as well (=> but i think is only the border of the line part, the line part is so thin that it looks like it is the whole line part that is displayed with the selected color)
When having a look at the properties of a new SequenceFlow drawn it seems that the fields BackgroundColor, BorderColor, Font color are empty... When you click in a field, there is only cursor blinking
Another question by the way after these tests : once you have change border color of a SequenceFlow, it is not possible to erase it then even if you empty the field it keeps the previous color set.
Actually it is also the same kind of behavior with the dash line part of a Text Annotation although color properties are set to black color (#000000)
HTH